In a mobile communication system using CDMA (Code Division Multiple access), power control plays a vital role in system performance. Power control is needed both in the uplink and downlink directions. On the uplink, the aim is to receive all signals in a base station with the same signal power. Without power control, mobiles close to a base station would dominate in signal strength mobiles located further away from the base station. In CDMA systems, signals sent by one base station are orthogonal with each other but reflections of signals and signals sent by other base stations cause interference, which is why power control is needed also in the downlink direction.
There are two types of power control: open loop and closed loop. In the open loop power control, power adjustments are based on measurements of the connection at the signal-receiving end. The performance of the open-loop power control depends on the duplex mode used in the network. In a network using TDD (Time Division Duplex), the outer loop control works well, since the transmitted and received signals are sent on the same frequency. In a FDD (Frequency Division Duplex) system, however, performance of the open-loop power control is not guaranteed, since the different frequencies used for transmitting and receiving signals can be subject to different kind of interference. In the closed loop power control, the measurements regarding the transmission are sent from the receiver back to the transmitter, which can adjust its transmission power according to the received measurement information.
For instance in UMTS (Universal Mobile Telecommunication System), in the FDD mode fast closed-loop power control, i.e. inner loop power control is used. In this method, the signal-to-interference ratio is measured over one time slot, and the signal transmitting end is ordered either to increase or decrease its transmission power. In the outer loop power control method, the signal-to-interference ratio is adjusted by the base station, and the ratio is used in the fast closed-loop power control mechanism.
Basically, transportation of information in UMTS is performed basically using two types of channels, i.e. control channels and traffic channels. At the protocol level, the channels can be divided into physical channels, transport channels and logical channels. Transport channels are channels that define the radio interface transmission at the radio network layer and that are used between layer 1 and layer 2. Logical channels are used within layer 2. A transport channel defines how data is to be sent over the physical channels, whereas a logical channel defines the type of data to be sent.
Terminal equipment can simultaneously have several dedicated connections active, for instance some connections for real-time speech transmission and some for non-real-time data transmission. For instance, in the UMTS system's FDD mode on the uplink, this would result in having several DPDCH (Dedicated Physical Data Channel) channels in the physical layer, which channels are mapped onto corresponding transport channels for the transport network layer transmission. The transport channels are conveyed by transport bearers, each of which typically transmits its own power control information in the form of indicators, such as CRCI (Cyclic Redundancy Check Indicator) and QE (Quality Estimate). In the known solutions, the power control information is transported in the transport bearers real-time, not taking into account whether the corresponding transport channel is allocated for real-time or for non-real-time information.
The current solution for transporting power control information is poorly suited for a normal usage situation of a mobile terminal, where real-time and non-real-time applications can be active at the same time. The current solutions are not optimized for the usage of an important resource in a telecommunication network, i.e. bandwidth.